The commonly used existing cargo inspection system, which employs radiography technology, generally causes a single energy radiations to interact with the inspected object, detects the radiations which have interacted with the inspected object, and then obtains an image. Such a system is capable of reflecting the change in shape and mass thickness of the inspected object. However, it can't identify materials in the object.
It is well known that when X-rays with different energy levels interact with an object, the resultant physical reaction is related to the material attribute of the object. Thus, the interaction between X-rays and the same material varies with the energy change of X-rays. The probability with which photoemission, Compton and electron pair effects would occur predominate respectively in different energy spectra. The three physical effects also correlate to the atomic number of the material.
The specific interaction relationship between X-rays and object is expressed by Equation (1):
                                          μ            m                    ⁢                      t            m                          =                  -                      ln            ⁡                          (                                                I                  ′                                                  I                  0                                            )                                                          (        1        )            where tm represents mass thickness of the object, μm represents equivalent mass attenuation coefficient of this energy spectrum for X-rays and is related to materials of the object and radiation energy, I′ represents the intensity value of X-rays having certain energy after interacting with the object under inspection, and I0 represents the intensity value of the X-rays having certain energy before interacting with the object.
It is obvious that the respective influences from the material and the mass thickness of the object can't be distinguished simultaneously from each other if only using the radiations having a single energy. However, it is possible to obtain the probability that an associated physical reaction has occurred between X-rays having different energy levels and the material by detecting the resultant X-rays after X-rays interacts with the object, and thereby determining the material attribute of the object. In small-sized luggage inspection system, the identification for the material of object under inspection is realized by use of X-rays with two different energy levels. However, the energies of X-rays used in such a system are not sufficient to penetrate through items of large mass thickness at all, and thus this approach is inapplicable to examining large or medium-sized objects, such as containers and air containers.
Several years ago, Patent Document 1 (U.S. Pat. No. 5,524,133) proposed a technical concept that two high-energy X-ray beams with different energy levels are used to recognize the material attribute of large-sized objects. In such a system, two sets of fixed X-ray sources and two corresponding groups of detector arrays are provided. The two X-ray machines provide two X-ray beams with different energy levels, where one of the energy levels is higher than the other. For example, one of the energy levels is 5 MeV and the other is 1 MeV. Then, the average atomic number of the material is determined by looking up a pre-created look-up table based on the ratio between the two detection results. Due to the problems of complex structure, expensive cost, etc. caused by two sets of X-ray machines and two groups of detector arrays, this method has not been widely applied since it is disclosed in 1993.
In order to overcome the problems of complex structure, expensive cost, etc., Patent Document 2 (WO 00/437600) and Patent Document 3 (U.S. Pat. No. 6,069,936) proposed to obtain two X-ray beams by modulating the X-ray beams generated from one accelerator through a filter. Both of technical concepts of Patent Document 2 and Patent Document 3 are to use a single accelerator to obtain X-ray beams having different energy levels. However, the two X-ray beams obtained by filtering the X-ray beams using the filter mounted on the accelerator have limited difference in their energy spectra, thereby restricting the scope for an accurate material identification.
Patent Document 4 (WO 2004/030162 A2) proposed an approach of obtaining dual-energy X-rays based on traveling wave LINAC. It demonstrates the feasibility to obtain two X-ray beams with different energy levels by means of a single accelerator.
Patent Document 5 (WO 2005/084352 A2) discloses that a high and dual-energy method is utilized to detect the object containing high-Z (high atomic number) materials. In Patent Document 5, a statistical function is given, and a threshold is adjusted based on the selected standard variance for balancing sensitivity and accuracy, and further warning of the high-Z material whose atomic number is higher than a preset value.
All of the above-mentioned methods are the material-identifying method by use of radiations having two different energy levels, in which the determination is made as to whether the object is suspect by computing from the detection results on the two different energy levels. In Patent Document 1 and Patent Document 5, the computation is performed in a manner of, for example, looking up a look-up table based on the ratio between the two detection results, so as to determine whether the object contains suspicious material. However, because of the limitation of the energy levels of two X-ray beams and existing detection errors, the chance is very high that a misjudgment occurs in this method when the object to be detected is seriously intermixed, or when the object has a small mass thickness. According to the judging method using the ratio between two detected values, the same function form is unable to distinguish the detected values' difference between different materials, and meanwhile, there exists probability that the ratios between different materials are the same. This will lead to an inaccurate detection result. Moreover, since the material range to which the two energy levels are sensitive is limited, it is impossible to accurately identify both the low and high-Z materials at the same time.